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Quotes - Engineering - SalesWhat a plant-size screw compressor asks of its dryer
High steady flow, a warm room and a load profile worth reading
A 60 to 250 hp screw delivers 260 to 1,150 SCFM and normally runs two or three shifts as the plant's main compressor. The dryers that serve it draw 2 to 7 kW, so the choice between non-cycling, cycling and water-cooled shows up on the power bill, and this page sets out how the horsepower becomes a dryer model.
At 60 hp and above the compressor is a fixed installation with a controller that logs loaded hours, an aftercooler that adds 15 to 20°F to the room temperature and a 2 to 3 in discharge. The dryer must carry the full delivered flow, reject 2 to 7 kW of heat somewhere, and hold 38°F at 100 percent on days and at 40 percent overnight.
60 to 75 hp
- 260 to 345 SCFM delivered
- Dryer size 300 to 400 SCFM
- Mid-size machine shops, plastics, packaging
100 to 125 hp
- 430 to 575 SCFM delivered
- Dryer size 500 to 600 SCFM
- Plants with a paint line or blast room on the header
150 to 200 hp
- 645 to 920 SCFM delivered
- Dryer size 800 to 1,000 SCFM
- Foundries, bottling, automotive suppliers
250 hp
- 1,075 to 1,150 SCFM delivered
- Dryer size 1,250 SCFM; 1,500 to 1,600 in a hot room
- The last single-dryer size before flanged headers
Horsepower to dryer size, 60 to 250 hp
Delivered flow at 100 to 125 psig for a single-stage screw at 4.3 to 4.6 SCFM per hp. Dryer sizes at the rating point; the hot-room column applies the 0.88 factor for a 110°F compressor room, which is where most of these machines live in July.
| COMPRESSOR HP | TYPICAL DELIVERY | DRYER SIZE AT 100°F | DRYER SIZE, HOT ROOM (110°F) | DRYER BAND |
| 60 hp | 260 to 275 SCFM | 300 to 325 SCFM | 325 to 400 SCFM | 300 to 1,250 SCFM |
| 75 hp | 325 to 345 SCFM | 400 SCFM | 400 to 450 SCFM | 300 to 1,250 SCFM cycling |
| 100 hp | 430 to 460 SCFM | 500 SCFM | 550 to 600 SCFM | 300 to 1,250 SCFM |
| 125 hp | 540 to 575 SCFM | 600 to 635 SCFM | 650 to 800 SCFM | 300 to 1,250 SCFM cycling |
| 150 hp | 645 to 690 SCFM | 800 SCFM | 800 to 850 SCFM | 300 to 1,250 SCFM |
| 200 hp | 860 to 920 SCFM | 1,000 to 1,050 SCFM | 1,050 to 1,250 SCFM | 300 to 1,250 SCFM cycling |
| 250 hp | 1,075 to 1,150 SCFM | 1,250 SCFM | 1,500 to 1,600 SCFM | 1,600 to 2,000 SCFM |
Non-cycling: the plant runs around the clock
Three shifts and a compressor loaded 70 percent of the time mean the dryer would rarely get to rest anyway. A direct-expansion unit with a hot-gas bypass gives the flattest dew point for the least money, and the previous-generation plant dryers cross-reference to it size for size.
- DXR Series: Direct-expansion non-cycling dryer in 27 sizes from 10 to 4,200 SCFM; the widest single range on the site, with a full data sheet behind every model.
- HPRN Series: Non-cycling refrigerated dryer from 75 to 1,200 SCFM for plant air on a single compressor; previous generation, cross-referenced to current models.
Cycling: one or two shifts, or a trim compressor
Where the header drops below half load for a third of the day, a cycling dryer earns back its premium in one to two years at this size. Phase-change, glycol and dry-mass designs all cover 300 to 1,250 SCFM, and the digital-scroll units modulate continuously from 600 SCFM up.
- FLXA Series: Current cycling dryer, 75 to 2,000 SCFM, air- or water-cooled, with low-GWP refrigerants and a heat-transfer cycling control that follows the load.
- TMC Series: Thermal-mass cycling dryer from 30 to 635 SCFM; stores cold in a thermal mass so the refrigeration compressor rests when air demand drops.
- NXC Series: Cycling dryer in 19 sizes from 20 to 2,000 SCFM with a dry thermal mass and up to 80 percent power savings at part load.
- AES Series: Digital-scroll cycling dryer from 600 to 10,000 SCFM in air- and water-cooled builds, matching refrigeration capacity to demand continuously.
Installing a 300 to 1,250 SCFM dryer
At this size the dryer is a piece of plant, wired at 460 V three phase, piped at 2 to 3 in and rejecting several kilowatts into the room or the cooling-water loop.
Air-cooled dryers between 300 and 1,250 SCFM draw 2 to 7 kW and reject that, plus the heat they take out of the air, into a compressor room that is already carrying the compressor's own heat. If the room cannot be ventilated to hold 100°F, duct the condenser discharge out or specify the water-cooled build, available from 800 SCFM in the cycling line and throughout the digital-scroll range; on 85°F tower water it also draws a quarter to a third less power.
Pipe it after the receiver with a 1 micron coalescing prefilter ahead and a 0.01 micron oil-removal afterfilter behind, each sized for the compressor's full output, and a three-valve bypass around the set. Connections step from 2 in NPT at 300 SCFM to 3 in at 850 and above; the dryer outlet should never be the smallest pipe in the system. Fit zero-loss drains on the separator, dryer and both filter bowls.
A 100 hp screw is the 500 SCFM dryer at 100°F; in a 110°F room with a tired aftercooler delivering 115°F air it becomes the 800.
Size a dryer for my plant compressor
Running costs and sizing at 60 to 250 hp
In the room, on top of the compressor's own heat. A dryer in this range draws 2 to 7 kW and its condenser also rejects all the heat it removed from the compressed air, so a room with marginal ventilation warms by several degrees and every machine in it suffers. Duct the condenser discharge outside, add ventilation sized for both loads, or take the water-cooled build where tower water exists.
Because every restriction downstream of the compressor is paid for in discharge pressure. A dryer piped down to a smaller line than the compressor discharge adds pressure drop at full flow, and the compressor answers by running to a higher setpoint all day. Connections here step from 2 in NPT at 300 SCFM to 3 in at 850 SCFM and above; keep the header at least that size.
Size it for the bigger of the two, since either may be the one running. The 125 hp alone delivers 540 to 575 SCFM and would take the 600 to 635 SCFM dryer, but the day the 150 hp covers for it the dryer sees 645 to 690 SCFM and needs the 800. Buying the smaller dryer leaves the plant wet only on the days the main machine is down, which is exactly when nobody is watching the dew point.
Yes, from 600 SCFM up, which takes in the 125 hp machine and everything above it on this page. A digital-scroll unit modulates its refrigeration capacity continuously instead of starting and stopping, so it follows a header that swings between full load and partial flow with a steadier dew point than an on-off design. Below 600 SCFM the phase-change, glycol and dry-mass cycling designs cover the duty.
Yes, because the receiver only removes liquid. Air leaving it still carries oil aerosol from the lubricated screw and rust from the tank walls, and both settle on the dryer's exchanger. A coated exchanger transfers heat poorly, and the plant sees it as a slow rise in pressure drop across the dryer and a dew point that no longer holds in summer. A 1 micron coalescing prefilter sized for the compressor's full output catches that load where an element change takes minutes.
Yes. Its hot-gas bypass keeps the evaporator just above freezing accross that load range, so the dew point stays as steady at 40 percent load as it does at 100. The cost is power, since the refrigeration compressor runs at full draw through every quiet hour. That is the trade a cycling dryer removes, and why the loaded-hours log, not the dew point, decides between the two.
